Method and tool for swaging internal threads



March 8, 1955 Q J. J. BARTH 2,703,419

METHOD AND TOOL FOR SWAGING INTERNAL THREADS Filed March 6, 1951 sSheets-Shet 1 I lllllll INVENTOR. JOHN J BA RTH BY I A TT'OENEXS.

' March 1955 J. J. BARTH 2,703,419 7 METHOD AND fOOL FOR SWAGING INTERNAL THREADS Filed March 6, 1951 3 Sheets-Sheet 2 ENTOR.

, JOHN J. BARTH BY OMMQW HTTOENEY$ March 8, 1955 J, BARTH 2,703,419

METHOD AND TOOL FOR SWAGING INTERNAL THREADS Filed March 6, 1951 3Sheets-Sheet 3 V ugll llllll INVENTOR. I JOHN J.BARTH 74 73 BY M 70A'rro QNEXS.

United States Patent METHOD AND TOOL FOR SWAGING INTERNAL THREADS JohnJ. Barth, Lakewood, Ohio, assignor to The Barth Corporation, acorporation of Ohio Application March 6, 1951, Serial No. 214,110

7 Claims. (Cl. -152) The present invention relates generally asindicated to a thread forming tool and method and article producedthereby, and more particularly to certain improvements in a tool forforming strong, deep, internal threads in sheet stock blanks orworkpieces which are too thin to have the desired number of threadsformed therein and which even though formed with tubular transverseextrusions of length corresponding to the desired number of threads,such extrusions are too thin-walled to permit forming of threads ofdesired depth with known types of thread-forming tools; in the method offorming such strong, deep, internal threads in thin work-pieces; and inthe internally threaded articles thus formed by the use of the tool andby the practice of the method.

Hitherto, except in instances wherein the thread formed as by aself-tapping screw in a thin work-piece is adequate, it has beenproposed to form a plurality of internal threads in a thin w0rk-piece byfirst piercing and extruding the Work-piece to provide a transversetubular projection thereon and then cutting internal threads in suchprojection as with an ordinary tap, this usually being a multi-stageprocess involving use of separate piercing, extruding, and threadingtools and, of course, in most cases, the thinning of the wall of theprojection only permits cutting of very shallow threads which areconsiderably less than full depth. Accordingly, the increase in thenumber of threads made possible by the extrusion results in threads ofcorrespondingly less depth whereby the net gain in thread strength, ifany, is not appreciable. It has therefore been necessary to resort tothe employment of separate nuts having the requisite number of threadsof substantially full depth for threaded engagement with screwsextending through punched holes in the workpieces, and in the case ofnuts formed by drawing a cup of required depth, piercing the bottom ofthe cup to provide a tubular projection, and cutting threads in theprojection heavier stock is required and elaborate drawing equipment isrequired for stealing material from the area of the stock surroundingthe projection. Such drawn and tapped nuts as well as other forms ofnuts are not only of relatively high initial cost, but there is involvedin addition the cost of welding or otherwise securing the same to therelatively thinner work-pieces.

Accordingly, it is one primary object of this invention to provide acombination tool which in a single operation is capable of forming aplurality of substantially fulldepth internal threads in sheet stockblanks or Workpieces; to provide a thread-forming method which effectseconomies in labor and in material While at the same time the materialis subjected to plastic deformation or plastic flow to refine the grainstructure thereof and thereby impart superior strength characteristicsto the threaded portion and which results in the formation ofsubstantially full-depth threads although the wall thickness in thetubular projection would not permit the forming of such threads bypresently known methods; and to provide a sheet stock article having aninternally threaded tubular projection thereon characterized by thesuperiority of the threads therein by reason of their accuracy andsubstantial depth and the increased strength of the material resultingfrom plastic flow or displacement thereof.

More specifically stated, it is an object of this invention to provide acombination tool having a reduced end portion for forming a hole throughthe work-piece, an adjacent extruding portion for enlarglng the holewhile simultaneously extruding the material to form a transice versetubular extension or projection of length several times the thickness ofthe work-piece, and a threading portion operative to thread theextrusion preferably by plastic flow or deformation of the material ofthe projection.

It is a further object of this invention to provide a thread formingmethod which involves the steps of piercing a work-piece, extruding thework-piece to provide a transverse tubular projection, and forminginternal threads in such projection by the application of rotary slidingand wedging pressure therein to induce a plastic flow of the materialboth radially outwardly and inwardly which not only provides threads ofsubstantial depth but provides threads of great strength by reason ofthe refinement of the grain structure of the material and of the surfacehardening owing to plastic flow and ironing action.

It is a further object of this invention to provide a new article ofmanufacture characterized in that while the stock from which the articlewith tubular extrusions is made is too thin to provide an adequatenumber of threads of required depth, such requirements are neverthelesssatisfied without the preliminary expense of drawing or cupping,'asabove indicated, and of providing a deep draw stock when a less ductileor plastic stock might be preferred.

Other objects and advantages will become apparent as the followingdescription proceeds.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following description andthe annexed drawings setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principle of the invention may beemployed.

In said annexed drawings:

Fig. 1 is a side elevation view, on an enlarged scale, of one form ofthe combination tool constituting the present invention;

Fig. 2 is an end elevation of the tool as viewed from the bottom end ofFig. 1;

Figs. 3 and 4 illustrate modifications in the reduced end portion of thetool, Fig. 3 showing a drill and Fig. 4 showing an end mill, whereas inFig. 1 the reduced end portion constitutes a piercing punch;

Figs. 5, 6 and 8 are side elevation views of modifications in theintermediate extruding portion of the tool, Fig. 5 showing a sphericalextruding portion, Fig. 6 showing a tapered extruding portion of ovalcross section, and Fig. 8 showing a spherical extruding portion havingnoncutting flutes;

Figs. 7 and 9 are transverse cross section views taken respectivelyalong the lines 7-7 and 9-9 of Figs. 6 and '8 to show the oval crosssection and non-cutting flutes;

Figs. 10, 12, and 14 are side elevation and cross section views andFigs. 11, 13, and 15 are end elevation views as viewed from the bottomend of Figs. 10, 12, and 14, respectively, Figs. 12 and 14 being crosssection views taken along lines 12-12, Fig. 13 and 14-14, Fig. 15, toshow several modifications in the threading portion of the tool, Figs.10-11 showing a cutting tap, Figs. 12-13 showing a non-cutting flutedtap, and Figs. 14-15 showing a non-cutting'tap of generally oval crosssection;

Fig. 16 illustrates a further modification in the threading portion ofthe tool, a tapered thread being shown betwelen the straight thread andthe extruding portion of the too Fig. 17 shows a tapered threadpartially axially overlapping the extruding portion of the tool;

Fig. 18 shows a straight threading portion chamfered so as to besubstantially tangent to the extruding portion;

Figs. 19, 20 and 21 show the successive steps in the formation of aninternally threaded article, Fig. 19 showing the piercing step, Fig. 20showing the extruding step, and Fig. 21 showing the completed internallythreaded article; and

Fig. 22 is a fragmentary radial cross section view showing the threadsin the article of Fig. 21 enlarged several times.

Broadly stated, one aspect of this invention consists in the provisionof a combination tool which has a reduced end portion optionally in theform of a piercing punch, a drill, end mill, or the like, which incooperation with a die having a larger orifice than such tool endportion is designed to pierce the work-piece which is to be internallythreaded, the tool being fed axially and rotated in the case of thedrill or end mill and fed axially and preferably, but not necessarily,rotated in the case of the piercing punch; which, adjacent to theaforesaid reduced end portion, has an intermediate portion of spherical,conical, or other progressively enlarging shape and preferably ofgenerally oval cross section or at least with rounded edges operativeupon axial feeding and rotation of the tool (rotation not required whenintermediate portion is of circular cross section) to enlarge the holein the work-piece and to displace the metal to provide a transversetubular projection of length several times the thickness of thework-piece and necessarily of wall thickness less than the thickness ofthe work-piece; which, adjacent to or overlapping such intermediateportion, and also preferably of generally oval cross section or withrounded corners, is formed with a threaded portion de signed, upon axialfeeding and rotation of the tool, to cause a plastic flow or deformationof the material of the work-piece extrusion without cutting toprogressively form internal threads in the proiection; and which.adjacent to the aforesaid threaded portion has finishing threads toeither continue the deformation without cutting action or, if desired.the last-mentioned portion may be designed to cut in order to accuratelysize the threads which have been formed by plastic flow of the material.

In essence, the tool comprises three portions. namely a piercingportion. an extruding portion, and a threading portion, and in view ofthe several modifications which may be made in each portion, ashereinafter more particularly set forth, a multitude of different toolsmay be provided in accordance with the thickness and physical propertiesof the material of the work-piece, and it is to be noted that each toolis capable of forming internal threads in the work-piece in one simpleoperation.

A second aspect of the present invention, also broadly stated. concernsthe method of forming internal threads in a sheet stock blank whichcomprises piercing, extruding, and threading the extrusion in a mannerto achieve the required number of substantially full-depth threads ofgreat strength.

A third aspect of the present invention, in its broad sense, resides inthe internally threaded article as such which is provided with atransverse tubular projection having substantiallv full depth, strongthreads contrary to what could be obtained in a work-piece of comparablethickness and using conventional methods.

With the foregoing in mind, reference will now be made specifically tothe drawings, and first to Figs. 1 and 2 in which the tool 1 comprises acylindrical piercing punch 2. an intermediate s herical extrudingportion 3 smoothly ioined as at 4 to the punch 2, a threaded portion 5having a tapered pitch diameter and chamfered as at 6 to smoothlv jointhe extruding portion 3. a straight thread portion 7 adiacent the largeend of the tapered thread portion 5, a shank 8, and a tang 9 by whichthe tool is rotated by a tapping machine, a drill press, or like machinetool with or without a conventional reverse gear tap attachment. Asapparent, the V-threads 5 and 7 herein shown are to be re arded asmerely exemplary and obviously o her well known forms of threads may besubstituted without de arting from the spirit of the invention.

As best shown in Fig. 2, the extruding and threaded portions 3 and 5, 7of the tool are of generally oval cross section providing ribs or lobesof convexly curved form which resemble gear teeth whereby upon relativerotation and axial feeding of the tool 1 and a work-piece a transversetubular extrusion is formed on the latter by a plastic flow ordeformation of the material operated upon bv axial and radial forceswhich in this case act onlv at diametrically opposed regions of thepierced work-piece whereby to reduce the power reuuired to rotate thetool. By reason of the oval cross section of the threaded portions 5 and7, threads in the extruded tubular proiection of the work-piece areprogressively formed by plastic flow and under conditions of low torqueon the tool. Moreover, the ribs and lobes of said portions 3 and 5, 7 ofthe tool effect a rotary sliding or ironing and wedging pressure wherebyto surface harden the work-piece operated u on.

One convenient manner of forming the generally oval cross section is toprovide a threaded cylindrical tool blank having parallel flat sides 10,the corners 11 of which have been rounded as by grinding with a wheelhaving a periphery which is concavely curved in radial cross section toa radius 12. Accordingly, upon relative movement of the wheel and toolblank along an angle corresponding to the angle of the pitch line of thetapered thread 5, the sharp corners 11 will be rounded and, of course,along the straight thread portion 7 the relative movement between thewheel and tool blank will be along a path parallel to the axis of thetool 1. The edges 13 may be broken or not, as desired, there not beingany cutting action thereof during the infeed movement of the tool ineither event or shaving by such edges 13 during the outfeed or reverserotation of the tool.

In using tool 1, the same is rotated or not, as desired, and movedaxially to engage a work-piece, the reduced end portion 2 punching out adisc of material from the work-piece. As the tool is axially advanced atdesired rate and now rotated the extruding portion 3 enlarges the holein the work-piece and causes a plastic flow or deformation of thematerial to form a transverse tubular extrusion or projection on thework-piece and then as the threaded portion 5 engages the work-piece theworking of the material is continued to form internal threads and atthis stage of advance of the tool, the tool is selffeeding whereby theuse of a tapping machine with a lead screw or cam mechanism for feedingin accordance with the pitch of the threads 5 and 7 is not required,although the same is desirable in order to avoid application ofexcessive axial force and thus causing the tool to operate as a broach.If desired, the threads 7 at the upper end may be designed to cutwhereby to accurately size the threaded tubular projection in thework-piece.

Having thus described in detail one embodiment of the tool, referencewill now be made to Figs. 3-18 which depict various modifications in theseveral working portions of the tool, namely the reduced end piercingportion, the extruding portion, and the threading portion.

As previously indicated, and as shown in Figs. 3 and 4, the reduced endportion of the tool instead of constituting a piercing punch 2 as inFig. 1 may be a drill 21 as shown in Fig. 3 or an end mill 22 as shownin Fig. 4. In the use of a tool having a reduced end portion in the formof an end mill 22, a cylindrical recess will be cut into the work-pieceblank to about one-half the thickness of the blank and the axial feedingpressure will shear out the remaining portion when the end mill isrotated and fed axially at recommended cutting speed. As evident, thereare herein shown three optional forms of reduced end portions 2, 21 and22 and any one may be selected according to the dictates of the user ofthe tool and any special circumstances which may render one formpreferable to the others. For example, it has been found that somework-pieces may not be conveniently punched and that drilling or millingproduces a hole which is not so apt to split upon expansion, andsimilarly other stocks may not be particularly suited for drilling ormilling for best results in the final product.

Typical alternatives for the spherical extruding portion 3 of ovaltransverse cross section as in Fig. 1 are shown in Figs. 5-9, the firstone 25 in Fig. 5 being spherical and of circular cross section andoperative to extrude the pierced work-piece whether or not the tool isrotated while fed axially; the second one 26 in Figs. 6 and 7 beingfrusto-conical and of generally oval cross section; and the third one 27in Figs. 8 and 9 being of spherical form, but instead of being of ovaltransverse cross section as in Fig. 2, the same is provided withnon-cutting flutes 28 which yet define ribs or lobes of convexly curvedorm.

Accordingly, with the several modifications in the reduced end portionand intermediate enlarged extruding portion as disclosed above, manydifferent combinations may be made up including additional ones withstill different reduced end portions or varied extruding portions ofdifferent shape in elevation and including three or more ribs or lobeswhere three or more flats or flutes are provided.

With respect to the alternatives for the threading portions 5 and 7 ofthe tool 1, several are shown in Figs. 10-18.

In Figs. 10 and 11 the threading portion is similar to that in a cuttingtap wherein there are a plurality of flutes 31 which form cutting edges32 and shaving edges 33 upon outfeed if no radial relief is provided.Thus, with a cutting tap as shown in Figs. 10 and 11, the pierced andextruded projection of the work-piece will have internal threads cuttherein and therefore the use of this modification will more or less berestricted to instances wherein the extrusion is relatively thick so asto permit formation of threads of desired depth or to instances whereshallow threads in relatively thin-walled extrusions are notobjectionable.

In Figs. 12 and 13 the threaded portion 35 of the tool is fluted as at36 with the corners 37 rounded or broken to form ribs or lobes ofconvexly curved form so as to avoid any cutting action as the tool isfed axially and rotated in the transverse projection of the work-piece.Of course, if desired the last few threads at the upper end of thenon-cutting tap in Fig. 12 may be arranged as in Fig. 10 to cut and thusaccurately size the threads which have been formed by non-cuttingplastic flow action.

In Figs. 14 and 15 there is shown a tool having a noncutting threadedportion 40 of generally oval cross section to eliminate cutting actionand, as indicated with respect to Fig. 12, the threads at the upper endof Fig. 14 may be arranged for cutting to accurately size the threadsformed by the non-cutting threads. In Fig. 14 the threads 40 start atthe largest portion of the extruding portion whereas in Fig. 1 thethreads 5 axially overlap the extruding portion 3.

In Fig. 16 a tapered thread similar to a tapered pipe thread is providedbetween the straight threaded portion 46 and the extruding portion ofthe tool whereby as the Work-piece is being extruded, portions of thematerial will be progressively displaced radially outwardly while thein-between portions of the material will be caused to flow radiallyinwardly into the valleys 47 between successive crests of the taperedthreaded portion 45. Here again the cross section of the tapered threadportion is preferably of non-cutting form such as in Figs. 13 and 15,for example.

In Fig. 17 the tapered threaded portion 50 is similar to thatillustrated in Fig. 16 except that such tapered threads axially overlapthe extruding portion of the tool to simultaneously extrude thework-piece and to start the formation of threads therein.

In Fig. 18 the straight thread portion of the tool is chamfered as at 56so as to be tangent with the extruding portion, the threads accordinglybeing truncated whereby as in the other forms having tapered or taperedand chamfered threads, the tool will feed itself in accordance with thepitch of the threads 55 whereby it is not required to use the tool in atapping machine or like machine having a lead screw or cam mechanism forfeeding.

In using the tool herein disclosed the sheet stock workpiece or blank 60is supported on a die 61 having an opening 62 larger than the majordiameter of the thread to be formed. The size of such opening 62 may bevaried, but should be at least equal to the major diameter of thethreads to be formed plus two times the minimum wall thickness betweenthe outside diameter of the extruded tubular projection 63 and the majordiameter of the threads. The tool, herein the tool 1 as illustrated inFig. 1, is disposed coaxially with the opening 62 and when fed axiallywith or without rotation in the case of the piercing punch 2 and withrotation in the case of the drill 21 and end mill 22, a hole 64 isformed in the blank and the surrounding portion of the blank is deformedto dish or frusto-conical shape about the peripheral edge 65 of theopening 62 as best shown in Fig. 19. As the tool 1 is continued to befed axially, the dish shape is deformed to generally spherical orconical form and then finally to generally cylindrical form and in thecase of the tool illustrated in Fig. l, for example, extrusion andcommencement of the forming of threads occurs simultaneously 6 pitch ofthe threads 5 and 7. The intermediate stage of the formation of theblank 60 is shown in Fig. 20.

After the tool 1 has been run through the blank, the internally threadedtransverse tubular projection 63 will have a form such as illustrated inFig. 21. Obviously, if the die hole 62 were somewhat larger, thecorrugations 67 on the outer surface of the projection 63 would be morepronounced and, of course, if the die hole 62 is smaller in diameter,such corrugations 67 would be hardly visib e.

Since no material of the blank 60 has been cut away other than the smalldiameter slug which is punched out by the piercing punch 2 or thematerial removed by drilling or milling with portions 21 or 22, all ofthe material of the extrusion 63 is used in providing a fuller threadthan otherwise obtainable.

Now with reference to Fig. 22, the dimension line 70 shows the diameterof the die hole, the dimension line 71 shows the major diameter of thethread, the dimension line 72 shows the minor diameter of a full thread,the dimension line 73 shows the inside diameter of the extrusion 63 orthe outside diameter of the extruding portion of the tool, and thedimension line 74 shows the inside diameter of an ordinary extrusion forconventional tapping. As evident, the progressive outward deformation ofthe material in applicants process by the crests of the threadingportion of the tool causes a radial inward flow of the metal to producea thread which is of substantially full depth without necessity ofhaving a corresponding thick wall in the extrusion 63.

By way of an actual example of the ordinary tapped extrusion, a blank 60of .020 stock was pierced with a hole of .073" diameter; formed with anextrusion of .080" length (including the .020 wall of stock) and of.200" outside diameter; and threads cut in the extrusion with a #10-32S. A. E. Regular V-thread tap. Following are the dimensions of thethreads and extrusion:

#10-32 threads:

.190 major diameter .1494 minor diameter .0203 full thread depthExtrusion (from volume of material .200" OD., .073"

ID., and .020" thickness):

.200 outside diameter .177 inside diameter .0115 wall thickness .080"length available for threading (including wall thickness of stock)Tapped extrusion (#10-32 threads cut with tap):

.005 wall thickness (from major diameter of thread to outside ofextrusion) 2% threads (.080/.O3l25) 32% thread (.0065/ .0203) Incontradistinction to the foregoing, the use of the present tool andmethod on the same .020" blank results in the formation of about 2%threads but such threads are of 60 to 75% full depth and greater becauseof the plastic flow and deformation of the material into the valleysbetween crests rather than cutting away material as is done with thetap. Similar results with different thicknesses of blanks, differentsizes of holes 64 in the blanks, and different thread sizes have beenobtained. With respect to the .073 diameter hole 64, this of course mustbe varied in accordance with the thickness and the ductility orplasticity of the particular material of the blank so as to avoidsplitting of the extrusion.

As shown in Fig. 22, the working of the metal by plastic flow ordeformation produces a grain structure which is generally sinuous inform thereby improving the strength characteristics of the material ascompared with cut threads, and moreover the rotary sliding and wedgingpressure irons smooth at least the major diameter and adjacent sidefaces of the threads and thus surface hardens the same while the minordiameter and adjacent side faces are of convex curved form in a radialplane, all surfaces being smooth and scratch-free.

The matter of selection of the particular form of reduced end portion 2,21, 22 or the like (as shown in Figs. 1, 3 and 4) according to thethickness and the properties of the material of the work-piece hasalready been discussed. However, the size of the portion 2, 21, or 22relative to the inside diameter of the extrusion 63 requiresconsideration, it being noted that a ratio 1:5 or smaller generallycauses splitting of the extrusion 63. Good results have been obtainedwith steel, aluminum alloy, brass, copper and like ductile metals usingratios of 1:2 to 1:3, the ratio in the specific example above being.073:.l58. On the other hand, while no splitting occurs when the ratiosare greater than 1:2, the extrusion 63 will not be of suificient lengthfor the desired number of threads.

With respect to the selection of one of the extruding portions 3, 25,26, and 27 as shown in Figs. 1, 5, 6-7 and 89 again the properties ofthe material of the work-piece will be controlling, it being noted,however, that extruding portions 3, 26, and 27 of oval cross section orof cross section defining ribs or lobes of convexly curved form exertpressure on the work-piece at circumferentially spaced areas and whenrotated effect not only a plastic fiow or displacement of the materialbut a deformation of the section of the extrusion and that such ovalextruding portions form a thicker and shorter extrusion 63 than does anextruding portion 25 of circular cross section.

In general, the maximum major axis of the extruding portion will be lessthan the pitch diameter of the threads to be formed and specifically ithas been found desirable to make such major axis approximately the sameas the tap drill diameter for a 75% thread whereby the formed threadswill be of substantial depth, often exceeding 75 In the trade, the tapdrill diameters are generally determined from either of the followingformulae:

Diameter of tap drill=(outside diameter of stud) (pitch of thread)=(outside diameter of stud)--(.75 X2 full thread depth) As to theselection of one of the threading portions -7, 30, 35, 40, 45, 50, and55 in Figs. 1, l0, 12, 14, 16, 17, and 18, respectively, any, exceptcutting threads 30, will be generally, but not necessarily, aspreviously explained, used where the wall-thickness of the extrusion 63is less than necessary for cutting or otherwise forming threads ofrequisite depth. Such non-cutting threaded portions 5-7, 35, 40, 45, 50,and 55 will produce fuller threads of greater strength and in actualtests the threads formed by plastic flow or deformation will strip thethreads of machine screws made of comparable material, this beingattributed to the cold-working of the material and surface hardeningthereof. In addition, since these threaded portions effect ironing andsurface hardening of the thread surfaces, severe stress between thethreads of the extrusion 63 and of the machine screw does not result inseizing or galling. However, as previously indicated, the threadedportions 5-7, 35, 40, 45, 50, and 55 may be of the cutting variety attheir upper ends to clean out and size the threads of the extrusion 63.

Another consideration in the selection of a particular threaded portionis that the provision of chamfered threaded portions 6 and 56 as inFigs. 1 and 18, and/or tapered threaded portions 5, 45, and 50, as inFigs. 1, 16, and 17, facilitates the starting of the threadingoperation, and once started the tool is self-feeding without unduestrain on the beginning portions of the threads. Moreover. the provisionof tapered threaded portions as in Figs. 1, l6 and 17, makes possiblethe formation of fuller threads and also reduces the power required todrive the tool in that the roots of the threads thereon are noteffective to deform the material of the work-piece outwardly but ratherto receive and to shape the material that flows thereinto as a result ofoutward deformation or displacement of the material worked on by thecrest portions of the threads. With imperfect threads, as on a startingtap, the threads are formed in the extrusion 63 by progressive outwarddeformation by the progressively narrowing truncated threads and byplastic flow of the material into the progressively widening valleys ortroughs of the threads on the tool.

A still further consideration in the selection of the threaded portionis that it is preferred to have an axial overlap of the threaded portionand extruding portion as in Figs. 1 and 17, for example, whereby themajor diameter of the beginning threads is performing the dual functionof extruding and commencing formation of threads in the incompletedextrusion whereby the material while being deformed outwardly is at thesame time moving inwardly into the valleys between the threads on thetool. Also in some cases the threading portion may have a slight taperto form threads which will tightly engage the machine screw threads toeliminate necessity of employing lock washers or the like.

Although the die hole 62 is not an integral component of the tool, itdoes have some bearing on the final product. Thus as previously stated,the minimum diameter of the die hole 62 is determined by the minimumwallthickness required around the major diameter of the internal threadsin the extrusion 63, and of course when the hole 62 is of somewhatlarger diameter than the minimum, the extrusion 63 will have morepronounced corrugations 67 on the outside surface thereof. Similarly,the provision of a sharp edge 65 around hole 62 may result in a burrprojecting above the top surface of the work-piece 60, a rounded edgewill result in a fillet between the extrusion 63 and the blank proper,and a beveled edge will produce a countersink which will facilitatestarting of a machine screw in the internal threads of the extrusion.

It has further been noted that good results have been obtained simply byproviding a pair of spaced supports instead of a die 61 with a hole 62.Also in forming threads in the wall of a tube, the piercing, extruding,and threading operations may be accomplished simply by supporting thetube on a flat or slightly concave surface.

In using the tool, a suitable lubricant will preferably be appliedthereto such as lard or sperm oil for steel and copper work-pieces, soapsolution for brass, copper, and steel work-pieces, petroleum jelly foraluminum workpieces, mineral oil emulsions or a mixture of flakedgraphite, beef tallow, and lard oil for steel work-pieces, etc. Thepresent invention has its greatest application for internally threadingthe extrusions 63 of sheet metal blanks 60, but obviously non-metallicblanks may be substituted. The principal requirement of the blanks isthat they be ductile and have sufiicient plasticity to permitdisplacement or plastic flow of the particles thereof without beingremoved from their sphere of attraction or to permit rather severedistortion or strain of the blank before rupturing.

In conclusion, the use of the present tool saves time and material,avoids necessity of separate fasteners, and makes possible fuller,stronger threads in sheet stock extrusions than can be obtained byordinary tapping processes in walls of the same thickness. Also, sincethe extrusions are of larger inside diameter than the minor diameter offull threads, the power required is less and the percentage of brokentools is materially reduced, this being common practice in ordinarymanufacturing where the tapping of soft materials results in tearing offof the threads to some extent if the tap drill diameter is too small.Likewise, the practice of the present method which induces plastic flowof the material and ironing thereof results in superior internallythreaded sheet stock articles having the desired number of threads ofdesired depth of greater strength and quality than tapped, ground,rolled, or milled threads. As to quality, it has been noted that thethread surfaces are hard and smooth and that class 1 or 2 screws have aclass 4 or tighter fit therewith by reason of the slight spring-back ofthe extrusion 63 which is strengthened and hardened as a result of theplastic flow or deformation of the material during the extrusion andthread-forming steps of the method.

Other modes of applying the principle of the invention may be employed,change being made as regards the details described, provided thefeatures stated in any of the following claims, or the equivalent ofsuch, be employed.

I therefore particularly point out and distinctly claim as my invention:

1. The method of forming internal threads in an opening of a work-pieceof wall thickness less than necessary for the requisite number ofthreads to be formed, comprising forming an integral tubular transverseprojection on the work-piece by plastic deformation of the materialsurrounding such opening radially outwardly and axially along a helicalpath and at circumferentially spaced areas, during the formation of suchprojection commencing the formation of internal threads by causingplastic deformation of the material of the work-piece radially outwardlyand inwardly along a helical path and at circumferential spaced areas,and then completing the formation of the internal threads by continuedplastic deformation as aforesaid.

2. A tool for forming internal threads in a relatively thin walledwork-piece having a hole therethrough comprising an extruding portion ofprogressively increasing size designed to enter the hole in thework-piece and to deform the material surrounding such hole to form atubular transverse projection in the work-piece upon relative rotationand axial movement of the tool and work piece, and a threading portionadjacent the large end of said extruding portion designed to formthreads in such tubular projection of the work-piece upon relativerotation and axial movement of said tool and the work-piece, saidthreading and extruding portions being provided with ribs therealongwhich are of convexly curved form in transverse cross-section to formsuch internally threaded projection by plastic flow of the material ofthe workpiece surrounding the hole, said threading and extrudingportions being partially axially overlapped whereby to be operativesimultaneously to form the tubular projection while threading of theprojection is commenced.

3. A tool for forming in a single operation internal threads in aworkpiece of wall thickness less than necessary for the requisite numberof threads to be formed, comprising a reduced end portion for forming anopening through the workpiece, an adjacent enlarged portion adapted toenlarge the opening in the workpiece while transforming the materialsurrounding theopening into a tubular projection having the requisitelength, and a threading portion adapted to form threads in theprojection upon relative rotation of the tool and the workpiece, saidenlarged portion and threading portion comprising a plurality of ribstherealong of convexly'curved transverse cross-section operative uponrelative rotation and axial feeding of said tool and workpiece to induceplastic deformation of the latter to the form of an internally threadedtubular projection without cutting'away of material and by pressureexerted at circumferentially spaced areas on the material surroundingthe opening of the workpiece, said enlarged portion and threadingportion being partially axially overlapped to commence formation ofthreads simultaneously with the formation of the tubular projection.

4. A tool for forming in a single operation internal threads in aworkpiece of wall thickness less than necessary for the requisite numberof threads to be formed, comprising a reduced end portion for forming anopening through the workpiece, an adjacent enlarged portion adapted toenlarge the opening in the workpiece While transforming the materialsurrounding the opening into a tubular projection having the requisitelength, and a threading portion adapted to form threads in theprojection upon relative rotation of the tool and the workpiece, saidenlarged and threading portions being partially overlapped axiallywhereby to'be operative simultaneously to form the tubular projectionwhile threading of the projection is commenced, and said enlarged andthreading portions being provided with ribs of convexly curved form intransverse cross-section whereby to form such internally threadedtubular projection in the work-piece by pressure Without cutting action,applied at circumferentially spaced points around the work-piece.

5. The tool of claim 4 wherein said threading portion where overlappedwith said enlarged portion includes chamfered threads.

6. The tool of claim 4 wherein said threading portion where overlappedwith said enlarged portion has threads of tapered pitch diameter.

7. The tool of claim 4 wherein said threading portion where overlappedwith said enlarged portion has chamfered threads of tapered pitchdiameter.

References Cited in the file of this patent UNITED STATES PATENTS ItalyDec. 23, 1938

